The present invention is directed to a xeroprinting process. More specifically, the present invention is directed to a xeroprinting process employing a migration imaging member and resulting in improved contrast potential. One embodiment of the present invention is directed to a xeroprinting process which comprises (1) providing a xeroprinting master comprising (a) a substrate and (b) a softenable layer comprising a softenable material, a charge transport material capable of transporting charges of one polarity, and migration marking material situated contiguous to the surface of the softenable layer spaced from the substrate, wherein a portion of the migration marking material has migrated through the softenable layer toward the substrate in imagewise fashion; (2) uniformly charging the xeroprinting master to a polarity opposite to the polarity that the charge transport material in the softenable layer is capable of transporting; (3) uniformly exposing the charged master to activating radiation, thereby discharging those areas of the master wherein the migration marking material has migrated toward the substrate and forming an electrostatic latent image; (4) developing the electrostatic latent image; and (5) transferring the developed image to a receiver sheet. Several advantages are associated with the process of the present invention, including increased contrast potential and suitability for development utilizing either dry toners or liquid toners.
In the art of printing/duplicating, various techniques have been developed for preparing masters for subsequent use in printing processes. For example, lithographic or offset printing is a well known and established printing process. In general, lithography is a method of printing from a printing plate which depends upon different properties of the imaged and non-imaged areas for printability. In conventional lithography, a lithographic intermediate is first prepared on silver halide film from the original; the printing plate is then contact exposed by intense UV light through the intermediate. UV exposure causes the exposed area of the printing plate to become hydrophobic; the non-exposed area is washed away by chemical treatment and becomes hydrophilic. Printing ink is then applied to the printing plate and the ink image is transferred to an offset roller where the actual printing takes place. Although lithographic printing provides high quality prints and high printing speed, the processes require the use of expensive intermediate films and printing plates. Additionally, considerable cost and time are consumed in their preparation, often requiring highly skilled labor and strict control measures. A further disadvantage is the time consuming process and difficulty in setting up the printing press to achieve the proper water to ink balance required to produce the desired results during the printing process. This results in further increased cost and delay time in obtaining the first acceptable print.
The above mentioned problems become especially severe in the manufacture of high quality color prints when several color separation images must be superimposed on the same receiving medium. Because of the high cost and complexities associated with the preparations of expensive printing plates and press runs, color proofing is employed to form representative interim prints (called proofs) from color separation components to allow the end user to determine whether the finished prints faithfully reproduce the desired results. As is often the case, the separation components can require repeated alteration to satisfy the end user. Only when the end user is satisfied with the results, a printing plate associated with each separation component is prepared and ultimately employed in the press run. An example of a color proofing system is the CROMALIN system, introduced by E. I. duPont de Nemours & Company in 1972 and widely used in the printing industry, and consisting of a light sensitive tacky photopolymer layer laminated to paper. The photopolymer layer is contact exposed through a color separation component under a UV source. The exposed areas polymerize and lose their tackiness, while the non-exposed areas remain tacky. Toners are then applied and adhere to the tacky areas. Since very different processes are employed in proofing and press runs, the proofs at best can only simulate the press sheets. Additionally, preparation of the color proofs is a time consuming process, and can require about 30 minutes per proof.
Xerographic printing is another well known printing technique. In conventional xerographic printing, an electrostatic image is first produced, either by lens coupled exposure to visible light or by laser scanning, on a conventional photoreceptor; the electrostatic image is then toned, followed by transfer of the toner image to a receiving medium. While this printing process offers the advantages of ease of operation and printing stability and requires less skilled involvement and labor cost, the combined requirements of high quality and high printing speed needed in commercial printing cannot be met easily at reasonable cost because, to provide high quality and avoid certain artifacts, very high-picture-element density is also required. If a new image were to be written, for example, on the photoreceptor for each print, these requirements for high speed and high density would imply electronic bandwidths and (if laser scanning were used) modulation rates and polygon rotation speeds which are very unlikely to be available at reasonable cost in the foreseeable future. In addition, the difficulties associated with conventional xerographic duplicating and printing include the necessity to repeat the imagewise exposure step continually at high speed.
Xeroprinting is another xerographic printing method. Conceptually, xeroprinting overcomes the above problems in a very simple way. Xeroprinting is an electrostatic printing process for printing multiple copies from a master plate or cylinder. The master plate can comprise a metal sheet upon which is imprinted an image in the form of a thin electrically insulating coating. The master plate can be made by photomechanical methods or by xerographic techniques. From the original, a single xeroprinting "master" can, for example, first be made slowly, in, for example, 30 to 60 seconds. This imaged material is typically an electrical conductor with an imagewise pattern of insulating areas made by photomechanical or xerographic techniques; it has different charge acceptance in the imaged and non-imaged areas. Thus, generally, the imaging surface of the master plate comprises an electrically insulating pattern corresponding to the desired image shape and electrically conductive areas corresponding to the background. The xeroprinting master is then uniformly charged; the charge remains trapped only on the insulating areas, and this electrostatic image can then be toned. After toner transfer to paper and possibly cleaning, the charge-tone-transfer-clean process is repeated at high speed. In principle, then, it is possible with a xeroprinting process to retain much of the simplicity, stability and quality of the xerographic process without the need for repeated imagewise exposure. As an additional bonus, it may not be necessary to employ a cleaning step, since the same area is repeatedly toned. Moreover, conventional toners can be used, avoiding the problem of lack of color saturation which is encountered with comparable schemes employing magnetography. High contrast potential and high resolution of the electrostatic latent image are important characteristics that determine print qualities of documents prepared by xeroprinting. However, these prior art xeroprinting techniques can produce prints of inferior quality because an insulating pattern on a metal conductor cannot be fully and uniformly charged near its boundaries. As contrast potential builds up along the boundaries of the insulating pattern, fringing electric fields from the insulating image areas repel incoming ions from the charging device, which is usually a corona charging device, to the adjacent electrically conductive background areas. This results not only in low contrast potential but also in poor print resolution. Additionally, some xeroprinting processes require numerous processing steps and complex equipment to prepare the master and/or final xeroprinted product. Some xeroprinting techniques also require messy photochemical processing and removal of materials in either the image or non-image areas of the master.
In U.S. Pat. No. 3,574,614 (Carreira), a xeroprinting process is disclosed in which the xeroprinting master is formed by applying an electric field to a layer of photoelectrophoretic imaging suspension between a blocking electrode and an injecting electrode, one of which is transparent, the suspension comprising a plurality of photoelectrophoretic particles in an insulating carrier liquid, imagewise exposing the suspension to electromagnetic radiation through the transparent electrode to form complementary images on the surfaces of the electrodes (the light exposed particles migrating from the injecting electrode to the blocking electrode), transferring one of the images to a conductive substrate, uniformly applying to the image bearing substrate an organic insulating binder such that the binder thickness both within the image formed and the non-image areas ranges from 1 to 20 microns. The xeroprinting process consists of applying a uniform charge to the surface of the image bearing substrate in the presence of electromagnetic radiation to form an electrostatic residual charge pattern corresponding to the non-image areas (areas void of photoelectrophoretic particles), developing the residual charge pattern, transferring the developer from the residual charge pattern to a copy sheet, and repeating the charging, developing and transferring steps. Alternatively, the insulating binder can be intimately blended with the dispersion of the photoelectrophoretic particles prior to insertion of the liquid mixture between the electrodes. The areas from which photoelectrophoretic particles have migrated become insulating and capable of supporting an electrostatic charge. A major problem, however, is that insulating images supported directly on a conducting substrate cannot be charged close to the edges, because fringe fields drive incoming ions to the grounded substrate. Another disadvantage of these processes is that they require the use of a liquid photoelectrophoretic imaging suspension to prepare the master. Additionally, the master making processes are extremely complicated, entailing the removal of one of the electrodes, transfer of one of the complementary images to a conductive substrate, and application of an organic insulating binder to the conductive substrate. Such complicated master making processes are incovenient to the user and can adversely affect the print quality. They also require additional time to dry the image prior to use as a xeroprinting master.
Unlike the liquid photoelectrophoretic imaging suspension system described in U.S. Pat. No. 3,574,614, solid imaging members have been prepared for dry migration systems. Dry migration imaging members are well known, and are described in detail in, for example, U.S. Pat. No. 3,975,195 (Goffe), U.S. Pat. No. 3,909,262 (Goffe et al.), U.S. Pat. No. 4,536,457 (Tam), U.S. Pat. No. 4,536,458 (Ng), U.S. Pat. No. 4,013,462 (Goffe et al.), and "Migration Imaging Mechanisms, Exploitation, and Future Prospects of Unique Photographic Technologies, XDM and AMEN", P. S. Vincett, G. J. Kovacs, M. C. Tam, A. L. Pundsack, and P. H. Soden, Journal of Imaging Science 30 (4) July/August, pp. 183-191 (1986), the disclosures of each of which are totally incorporated herein by reference. Migration imaging members containing charge transport materials in the softenable layer are also known, and are disclosed, for example, in U.S. Pat. Nos. 4,536,457 (Tam) and 4,536,458 (Ng). In a typical embodiment of these migration imaging systems, a migration imaging member comprising a substrate, a layer of softenable material, and photosensitive marking material is imaged by first forming a latent image by electrically charging the member and exposing the charged member to a pattern of activating electromagnetic radiation such as light. Where the photosensitive marking material is originally in the form of a fracturable layer contiguous with the upper surface of the softenable layer, the marking particles in the exposed area of the member migrate in depth toward the substrate when the member is developed by softening the softenable layer.
The expression "softenable" as used herein is intended to mean any material which can be rendered more permeable, thereby enabling particles to migrate through its bulk. Conventionally, changing the permeability of such material or reducing its resistance to migration of migration marking material is accomplished by dissolving, swelling, melting, or softening, by techniques, for example, such as contacting with heat, vapors, partial solvents, solvent vapors, solvents, and combinations thereof, or by otherwise reducing the viscosity of the softenable material by any suitable means.
The expression "fracturable" layer or material as used herein means any layer or material which is capable of breaking up during development, thereby permitting portions of the layer to migrate toward the substrate or to be otherwise removed. The fracturable layer is preferably particulate in the various embodiments of the migration imaging members. Such fracturable layers of marking material are typically contiguous to the surface of the softenable layer spaced apart from the substrate, and such fracturable layers can be substantially or wholly embedded in the softenable layer in various embodiments of the imaging members.
The expression "contiguous" as used herein is intended to mean in actual contact, touching, also, near, though not in contact, and adjoining, and is intended to describe generically the relationship of the fracturable layer of marking material in the softenable layer with the surface of the softenable layer spaced apart from the substrate.
The expression "optically sign-retained" as used herein is intended to mean that the dark (higher optical density) and light (lower optical density) areas of the visible image formed on the migration imaging member correspond to the dark and light areas of the illuminating electromagnetic radiation pattern.
The expression "optically sign-reversed" as used herein is intended to mean that the dark areas of the image formed on the migration imaging member correspond to the light areas of the illuminating electromagnetic radiation pattern and the light areas of the image formed on the migration imaging member correspond to the dark areas of the illuminating electromagnetic radiation pattern.
The expression "optical contrast density" as used herein is intended to mean the difference between maximum optical density (D.sub.max) and minimum optical density (D.sub.min) of an image. Optical density is measured for the purpose of this invention by diffuse densitometers with a blue Wratten No. 94 filter. The expression "optical density " as used herein is intended to mean "transmission optical density" and is represented by the formula: EQU D=log.sub.10 [l.sub.o /l]
where l is the transmitted light intensity and l.sub.o is the incident light intensity. For the purpose of this invention, all values of transmission optical density given in this invention include the substrate density of about 0.2 which is the typical density of a metallized polyester substrate.
There are various other systems for forming such images, wherein non-photosensitive or inert marking materials are arranged in the aforementioned fracturable layers, or dispersed throughout the softenable layer, as described in the aforementioned patents, which also discloses a variety of methods which can be used to form latent images upon migration imaging members.
Various means for developing the latent images can be used for migration imaging systems. These development methods include solvent wash away, solvent vapor softening, heat softening, and combinations of these methods, as well as any other method which changes the resistance of the softenable material to the migration of particulate marking material through the softenable layer to allow imagewise migration of the particles in depth toward the substrate. In the solvent wash away or meniscus development method, the migration marking material in the light struck region migrates toward the substrate through the softenable layer, which is softened and dissolved, and repacks into a more or less monolayer configuration. In migration imaging films supported by transparent substrates alone, this region exhibits a maximum optical density which can be as high as the initial optical density of the unprocessed film. On the other hand, the migration marking material in the unexposed region is substantially washed away and this region exhibits a minimum optical density which is essentially the optical density of the substrate alone. Therefore, the image sense of the developed image is optically sign reversed. Various methods and materials and combinations thereof have previously been used to fix such unfixed migration images. In the heat or vapor softening developing modes, the migration marking material in the light struck region disperses in the depth of the softenable layer after development and this region exhibits D.sub.min which is typically in the range of 0.6 to 0.7. This relatively high D.sub.min is a direct consequence of the depthwise dispersion of the otherwise unchanged migration marking material. On the other hand, the migration marking material in the unexposed region does not migrate and substantially remains in the original configuration, i.e. a monolayer. In migration imaging films supported by transparent substrates, this region exhibits a maximum optical density (D.sub.max) of about 1.8 to 1.9. Therefore, the image sense of the heat or vapor developed images is optically sign-retained.
Techniques have been devised to permit optically sign-reversed imaging with vapor development, but these techniques are generally complex and require critically controlled processing conditions. An example of such techniques can be found in U.S. Pat. No. 3,795,512, the disclosure of which is totally incorporated herein by reference.
For many imaging applications, it is desirable to produce negative images from a positive original or positive images from a negative original (optically sign-reversing imaging), preferably with low minimum optical density. Although the meniscus or solvent wash away development method produces optically sign-reversed images with low minimum optical density, it entails removal of materials from the migration imaging member, leaving the migration image largely or totally unprotected from abrasion. Although various methods and materials have previously been used to overcoat such unfixed migration images, the post-development overcoating step can be impractically costly and inconvenient for the end users. Additionally, disposal of the effluents washed from the migration imaging member during development can also be very costly.
The background portions of an imaged member can sometimes be transparentized by means of an agglomeration and coalescence effect. In this system, an imaging member comprising a softenable layer containing a fracturable layer of electrically photosensitive migration marking material is imaged in one process mode by electrostatically charging the member, exposing the member to an imagewise pattern of activating electromagnetic radiation, and softening the softenable layer by exposure for a few seconds to a solvent vapor thereby causing a selective migration in depth of the migration material in the softenable layer in the areas which were previously exposed to the activating radiation. The vapor developed image is then subjected to a heating step. Since the exposed particles gain a substantial net charge (typically 85 to 90 percent of the deposited surface charge) as a result of light exposure, they migrate substantially in depth in the softenable layer towards the substrate when exposed to a solvent vapor, thus causing a drastic reduction in optical density. The optical density in this region is typically in the region of 0.7 to 0.9 (including the substrate density of about 0.2) after vapor exposure, compared with an initial value of 1.8 to 1.9 (including the substrate density of about 0.2). In the unexposed region, the surface charge becomes discharged due to vapor exposure. The subsequent heating step causes the unmigrated, uncharged migration material in unexposed areas to agglomerate or flocculate, often accompanied by coalescence of the marking material particles, thereby resulting in a migration image of very low minimum optical density (in the unexposed areas) in the 0.25 to 0.35 range. Thus, the contrast density of the final image is typically in the range of 0.35 to 0.65. Alternatively, the migration image can be formed by heat followed by exposure to solvent vapors and a second heating step which also results in a migration image with very low minimum optical density. In this imaging system as well as in the previously described heat or vapor development techniques, the softenable layer remains substantially intact after development, with the image being self-fixed because the marking material particles are trapped within the softenable layer.
The word "agglomeration" as used herein is defined as the coming together and adhering of previously substantially separate particles, without the loss of identity of the particles.
The word "coalescence" as used herein is defined as the fusing together of such particles into larger units, usually accompanied by a change of shape of the coalesced particles towards a shape of lower energy, such as a sphere.
Generally, the softenable layer of migration imaging members is characterized by sensitivity to abrasion and foreign contaminants. Since a fracturable layer is located at or close to the surface of the softenable layer, abrasion can readily remove some of the fracturable layer during either manufacturing or use of the imaging member and adversely affect the final image. Foreign contamination such as finger prints can also cause defects to appear in any final image. Moreover, the softenable layer tends to cause blocking of migration imaging members when multiple members are stacked or when the migration imaging material is wound into rolls for storage or transportation. Blocking is the adhesion of adjacent objects to each other. Blocking usually results in damage to the objects when they are separated.
The sensitivity to abrasion and foreign contaminants can be reduced by forming an overcoating such as the overcoatings described in U.S. Pat. No. 3,909,262, the disclosure of which is totally incorporated herein by reference. However, because the migration imaging mechanisms for each development method are different and because they depend critically on the electrical properties of the surface of the softenable layer and on the complex interplay of the various electrical processes involving charge injection from the surface, charge transport through the softenable layer, charge capture by the photosensitive particles and charge ejection from the photosensitive particles, and the like, application of an overcoat to the softenable layer can cause changes in the delicate balance of these processes and result in degraded photographic characteristics compared with the non-overcoated migration imaging member. Notably, the photographic contrast density can degraded. Recently, improvements in migration imaging members and processes for forming images on these migration imaging members have been achieved. These improved migration imaging members and processes are described in U.S. Pat. No. 4,536,458 (Ng) and U.S. Pat. No. 4,536,457 (Tam).
U.S. Pat. No. 3,574,614 (Carreira) discloses a process in which a layer of photoelectrophoretic imaging suspension is subjected to an applied electric field between a blocking electrode and an injecting electrode, one of which is transparent, the suspension comprising a plurality of photoelectrophoretic particles in an insulating carrier liquid, imagewise exposing the suspension to electromagnetic radiation through the transparent electrode to form complementary images on the surfaces of the electrodes (the light exposed particles migrating from the injecting electrode to the blocking electrode), transferring one of the images to a conductive substrate, uniformly applying to the image bearing substrate an organic insulating binder such that the binder thickness both within the image formed and the non-image areas ranges from 1 to 20 micrometers, applying a uniform charge to the surface of the image bearing substrate in the presence of electromagnetic radiation to form an electrostatic residual charge pattern corresponding to the non-image areas (areas void of photoelectrophoretic particles), developing the residual charge pattern, transferring the developer from the residual charge pattern to a copy sheet and repeating the charging, developing and transferring steps. Alternatively, the insulating binder can be intimately blended with the dispersion of the photoelectrophoretic particles prior to insertion of the liquid mixture between the electrodes. The areas from which photoelectrophoretic particles have migrated become insulating and capable of supporting an electrostatic charge.
U.S. Pat. No. 4,536,458 (Ng) discloses a migration imaging member comprising a substrate and an electrically insulating softenable layer on the substrate, the softenable layer comprising migration marking material located at least at or near the surface of the softenable layer spaced from the substrate, and a charge transport molecule. The migration imaging member is electrostatically charged, exposed to activating radiation in an imagewise pattern, and developed by decreasing the resistance to migration, by exposure either to solvent vapor or heat, of marking material in depth in the softenable layer at least sufficient to allow migration of marking material whereby marking material migrates toward the substrate in image configuration. The preferred thickness of the softenable layer is about 0.7 to 2.5 micrometers, although thinner and thicker layers can also be utilized.
U.S. Pat. No. 4,536,457 (Tam) discloses a process in which a migration imaging member comprising a substrate and an electrically insulating softenable layer on the substrate, the softenable layer comprising migration marking material located at least at or near the surface of the softenable layer spaced from the substrate, and a charge transport molecule (e.g. the imaging member described in U.S. Pat. No. 4,536,458) is uniformly charged and exposed to activating radiation in an imagewise pattern. The resistance to migration of marking material in the softenable layer is thereafter decreased sufficiently by the application of solvent vapor to allow the light exposed particles to retain a slight net charge to prevent agglomeration and coalescence and to allow slight migration in depth of marking material towards the substrate in image configuration, and the resistance to migration of marking material in the softenable layer is further decreased sufficiently by heating to allow nonexposed marking material to agglomerate and coalese. The preferred thickness is about 0.5 to 2.5 micrometers, although thinner and thicker layers can be utilized.
U.S. Pat. No. 2,576,047 (Schaffert) discloses a xeroprinting device and process in which, for example, an insulating pattern in image configuration coated on a metal drum is electrostatically charged and thereafter developed with developer powder. The resulting powder image on the insulating pattern is electrostatically transferred to a receiving member, and the insulating pattern is then cleaned and recycled.
U.S. Pat. No. 3,967,818 (Gundlach) discloses a duplicating system for producing collated copy sets for precollated information. A xeroprinting master can be utilized as a master scroll that can move in reverse directions. The master is electrostatically charged and developed and the resulting toner image is transferred to a receiving member.
U.S. Pat. No. 3,765,330 (Gundlach) discloses a xeroprinting system which utilizes a printing member comprising a conductive substrate having raised and recessed areas of the same material and a layer of electrically resistive material contacting the relief areas and spanning without touching the recessed areas. A uniform charge is applied to the printing member to form discharged areas where the resistive material contacts the relief areas and charged areas where the resistive material spans the recessed areas. The printing member is then developed and the developed image is electrostatically transferred to a transfer sheet.
U.S. Pat. No. 3,820,984 (Gundlach) and U.S. Pat. No. 3,648,607 (Gundlach), the disclosures of each of which are totally incorporated herein by reference, disclose a migration imaging system having a migration imaging member with a binder layer of softenable material wherein a mixture of electrically photosensitive and inert fusible particles is dispersed and an imaging process wherein the fusible particles are fused, thereby fixing the migrated image of the two types of particles. The imaged member is used as a lithographic printing master.
U.S. Pat. No. 4,101,321 (Levy et al.), the disclosure of which is totally incorporated herein by reference, discloses an imaging system wherein an imaged migration imaging member is provided and either the background or the image areas of the image are selectively reduced to a more transparent condition. The imaged member comprises a softenable layer containing agglomerable materials in both image and complementary background configurations. The imaged member is contacted with solvent vapors capable of softening the softenable layer and heated, thereby causing the agglomerable material to selectively agglomerate in one of either the background or image areas.
U.S. Pat. No. 4,883,731 (Tam et al.), the disclosure of which is totally incorporated by reference, discloses a xeroprinting process wherein the xeroprinting master is a developed migration imaging member wherein a charge transport material is present in the softenable layer. According to the teachings of this patent, the xeroprinting process entails uniformly charging the master to a polarity the same as the polarity of charges which the charge transport material is capable of transporting, followed by flood exposure of the master to form a latent image, development of the latent image with a toner, and transfer of the developed image to a receiving member. The contrast voltage of the electrostatic latent image obtainable from this process generally initially increases with increasing flood exposure light intensity, typically reaches a maximum value of about 45 to 50 percent of the initially applied voltage and then decreases with further increase in flood exposure light intensity. The light intensity for the flood exposure step thus generally must be well controlled to maximize the contrast potential.
U.S. Pat. No. 4,880,715 (Tam et al.), the disclosure of which is totally incorporated by reference, discloses a xeroprinting process wherein the xeroprinting master is a developed migration imaging member wherein a charge transport material is present in the softenable layer and non-exposed marking material in the softenable layer is caused to agglomerate and coalesce. According to the teachings of this patent, the xeroprinting process entails uniformly charging the master to a polarity the same as the polarity of charges which the charge transport material is capable of transporting, followed by flood exposure of the master to form a latent image, development of the latent image with a toner, and transfer of the developed image to a receiving member. The contrast voltage of the electrostatic latent image obtainable from this process generally initially increases with increasing flood exposure light intensity, typically reaches a maximum value of about 60 percent of the initially applied voltage and then decreases with further increase in flood exposure light intensity. The light intensity for the flood exposure step thus generally must be well controlled to maximize the contrast potential.
U.S. Pat. No. 4,853,307 (Tam et al.), the disclosure of which is totally incorporated herein by reference, discloses a migration imaging member containing a copolymer of styrene and ethyl acrylate in at least one layer adjacent to the substrate. When developed, the imaging member can be used as a xeroprinting master. According to the teachings of this patent, the xeroprinting process entails uniformly charging the master to a polarity the same as the polarity of charges which the charge transport material is capable of transporting, followed by flood exposure of the master to form a latent image, development of the latent image with a toner, and transfer of the developed image to a receiving member.
U.S. Pat. No. 4,407,918 (Sato) discloses electrophotographic processes and apparatus for preparing plural copies from a single image. A photosensitive member is described which includes an electrically conductive substrate, a first photoconductive layer applied on the substrate, a charge retentive insulating layer applied on the first photoconductive layer, and a second conductive layer applied on the charge retentive layer. The photosensitive member is uniformly charged to a negative polarity and exposed to visible light. An image of a document to be copied is projected while the photosensitive member is positively charged. The photosensitive member is then exposed to visible and ultraviolet light, thereby trapping latent charged images across the charge retentive layer.
U.S. Pat. No. 4,518,668 (Nakayama) discloses a method for preparing a lithographic printing plate. A light sensitive material comprising a light sensitive layer and a photoconductive insulating layer is imagewise exposed and processed to form an electrostatic latent image on the photoconductive insulating layer. The image is then developed by charged opaque developer particles. This developed image is used for contact exposure of the underlying light sensitive lithographic master layer.
U.S. Pat. No. 4,520,089 (Tazuki et al.) discloses an electrophotographic offset master comprising a base paper, one side of which is provided with a back coat layer made of sericite. Another side of the base paper is provided with a precoat layer of a photoconductor and an adhesive. The master is prepared by imagewise exposure of the photoconductor followed by subsequent development and fixation thereof.
U.S. Pat. No. 4,533,611 (Winkelmann et al.) discloses a process for preparing a planographic printing plate in which a charged image is produced on a photoconductive layer and dieletric film applied thereon. The image is then developed and transferred to the printing plate.
Many disadvantages can be associated with some of these prior art techniques. For example, some prior art xeroprinting techniques produce poor quality prints because of their poor resolution capabilities caused by fringing electric fields as explained above. Some xeroprinting processes require numerous processing steps and complex equipment to prepare the master and/or final xeroprinted product. Messy photochemical processing and removal of materials in either the image or non-image areas of the master are also required for some xeroprinting techniques. In some approaches an insulating image is formed on a "leaky" dielectric; that is, a substrate that will accept and retain charge for a time longer than the time charges are applied to each particular spot, but that discharges over a relaxation time shorter than the time between charging and developing the latent image. The fundamental problem with this approach is that most resistive ("leaky") dielectric films are sensitive to relative humidity, and someties to age and temperature as well. The relaxation time varies beyond acceptable tolerance limits, over the normally encountered range of relative humidity, temperature, and product life. These shortcomings are particularly detrimental for color printing/duplicating applications which require high quality, high resolution, and high speed.
In recent years, the use of computer technology has become increasingly widespread in the commercial printing industry. While this has resulted in greatly increased efficiency and productivity of the printing process, the benefits of computer technology have mostly been confined to the prepress operations such as text editing, composition, pagination and the like. In order to provide the high quality, high resolution and high printing speed, the dominant printing process is still off-set lithography, which is not compatible with computer technology because of the very low photosensitivity of conventional printing plates. Other printing technologies such as laser xerography, thermal printing, ionography, ink jet printing, magnetography, and the like are compatible with computer technology, but they do not satisfy the combined requirements of high quality, high resolution, and high throughput speed. Therefore, there continues to be a need for improved imaging members and improved processes of xeroprinting.
In addition, a need remains for xeroprinting processes wherein the masters exhibit improved contrast voltage or contrast potential. The terms contrast voltage and contrast potential generally refer to the difference in voltage between the image areas and non-image areas (or between the D.sub.max areas and D.sub.min areas) of the master subsequent to formation of the master, uniform charging of the master, and exposure of the charged master to light. Generally, the contrast potential of an imaging member is a function of the thickness of the softenable layer or of the combined thickness of the sofenable layer and the charge transport layer, adhesive layer, and/or charge blocking layer, if present. Higher contrast potentials are particularly desirable when the image is to be toned with a dry toner, since dry toners typically require higher contrast potentials than liquid developers for acceptable development of images, especially at high printing speeds of 15 inches per second and more. Although contrast potential can be increased by increasing the thickness of the softenable layer, the thickness of the imaging member generally is limited by the mechanical requirements of the imaging member, such as flexibility, adhesion of the softenable layer to the substrate, and the like, as well as cost and ease of manufacture. Furthermore, it is desirable that during the exposure of the master to uniform light during xeroprinting, a wide range of light energies or intensities can be employed while maintaining maximum contrast potential. Thus, there is a need for an improved xeroprinting process which overcomes the above noted disadvantages and also provides enhanced contrast potential and a broader range of acceptable light energies or intensities for the uniform light exposure step.